Samsung care center reaping us off

I am very frustrate. I bought a tablet last year. and so far the tablet has gone back to Samsung to be fixed about 5 times, first they replaced the mother board as it kept on switching itself off went to them a couple of times for this. The went back again because it was not charging, when I went to collect it, they said they replaced the pin on the charging part of the tablet. But while the consultant was showing me that the charging pin was replace and the tablet is working, it switched itself off while I was there he took it to the back and came back after 5 minutes saying that something might have been loose and now it was fixed. I took the tablet home to try and use it, I first tried to insert a different sim card and it kept on pushing it out whichever way I tried to put it in. Then I used the old sim that I always used for that tablet and it got stuck in there. I took it to the care centre and they sent me a quote of R2 700 to replace the mother board and when I asked why they said the pin where you put in the sim was broken by who no one knows. later on Rudolf called and said I must only pay R450 for them to replace the pin. Why must I pay that amount while on warranty?